Commutation in a conventional direct current motor is essentially a mechanical switching operation in which) brushes and a segmented commutator cyclically reverse current through the armature conductors in a sequence as a function of rotor position, and such commutation results in friction wear and sparking with attendant generation of radio frequency noise. In order to eliminate such defects, brushless D.C. motors have been developed provided with electronic commutation means for controlling the armature current in accordance with the rotational position of the rotor. Brushless D.C. motors are also known which employ a permanently magnetized rotor and wherein the stator windings are energized in a cyclical sequence through semiconductor power switches which are sequentially gated in accordance with the rotational position of the rotor. Variable speed synchronous motors are also known wherein a synchro coupled to the motor rotor derives phase-displaced sine wave control signals at a frequency proportional to motor speed which regulate a cycloconverter for energizing the stator windings, and the phase angle and magnitude of the control signals are varied as a function of motor speed to advance motor torque angle at higher motor speeds.
Prior art electronically commutated motors which use optical or magnetic rotor position sensors are, in general, unnecessarily expensive and complicated and have relatively low sensitivity and relatively high temperature drift. Known electronically commutated motors using magnetic rotor position sensors have problems with D.C. offsets, while those employing optical rotor position sensors have problems with dirt and vibration. Also, prior art brushless D.C. motors do not have the desirable torque and speed characteristics of conventional direct current motors, while known variable speed synchronous motors either require complicated and expensive synchros for generating phase-displaced sine wave control signals or necessitate complicated and expensive analog circuits.